The present invention relates to a deflection apparatus used in television receivers and others capable of receiving and reproducing television signals of various systems, and a control method thereof. More particularly, it relates to a technology for controlling the deflection frequency and deflection pulse adequately in the transient period of changing from one television system to other system during receiving a signal, and at the time of on/off switching of power source of the receiver.
As a conventional deflection apparatus, for example, a configuration disclosed in Japanese Laid-open Patent No. 4-29471 has been proposed.
FIG. 12 is a block diagram of a conventional horizontal deflection apparatus. In FIG. 12, a frequency-to-voltage (F/V) converter 1 is a circuit for issuing a voltage (V) depending on the frequency (F) of an input horizontal synchronizing (sync) signal. A pulse width modulation (PWM) circuit 11 is a circuit for issuing a control pulse synchronized with the input horizontal sync signal, and controlled in pulse width by the output of the F/V converter 1.
A transistor 12 turns on or off the supply voltage (+B) by the control pulse of the PWM circuit 11, and generates and outputs a desired driving pulse voltage. A smoothing circuit 13 supplies a DC voltage Va obtained by smoothing and rectifying the driving pulse voltage as a power source of a horizontal deflection output circuit 4 through a choke coil 5.
The horizontal deflection output circuit 4 has the following configuration.
Between the collector and the emitter of a horizontal output transistor 10, a series circuit of deflection coil 6 and S correction capacitor 7, and a resonance capacitor 8 and a damper diode 9 are connected in parallel. The choke coil 5 is connected to the collector of the transistor 10, and the emitter is connected to the ground.
The operation is described below. In FIG. 12, when the frequency of the input horizontal sync signal (hereinafter called input frequency) is changed, the output voltage of the F/V converter 1 varies. Depending on the output voltage, the pulse width differs in the driving pulse voltage output from the PWM circuit 11. By the change of the pulse width of the driving pulse, the output voltage Va of the smoothing circuit 13 varies. Thus, as the voltage Va input into the horizontal deflection output circuit 4 through the choke coil 5 is controlled depending on the change of the input frequency, the current flowing in the deflection coil 6 becomes constant. As a result, if the input frequency changes, the current is controlled so that the display amplitude may be constant.
In the horizontal deflection circuit used actually, in addition to the above circuit, an oscillator 2 (indicated by bold dotted line in FIG. 12) capable of synchronizing with the horizontal sync signal is connected to the base of the horizontal output transistor 10. When a horizontal sync signal is not input in to the horizontal deflection circuit, the oscillator 2 oscillates freely and a raster is always displayed on the screen. When a horizontal sync signal is input into the horizontal deflection circuit, a synchronized picture is displayed.
The Digital Terrestrial Television Broadcasting in the United States is presented in 18 different systems, and each system significantly differs in the frequency of the horizontal sync signal. That is, the frequency ranges from 12.58 to 45.05 kHz. Accordingly, the oscillator 2 is mostly designed to synchronize by changing over the,oscillation frequency depending on the input frequency. This oscillator generates a signal synchronized with the horizontal-sync signal by dividing the oscillation frequency of the oscillator, and it is applicable to various television systems by changing over the frequency dividing ratio.
When the input frequency is changed from a high frequency to a low frequency at point A of the waveform in FIG. 13a, the oscillation frequency is changed over suddenly to the waveform in FIG. 13b according to the changeover signal. On the other hand, the output voltage Va of the smoothing circuit 13 falls slowly as shown in FIG. 13c. The horizontal deflection output circuit is operating at low frequency, but Va still remains high. Therefore the deflection pulse becomes high as indicated by solid-line waveform enclosed by the circular line in FIG. 13d. This pulse is added between the collector and emitter of the w horizontal output transistor. Hence it is necessary to control the oscillation frequency as shown by the dotted line in FIG. 13b. 
The oscillation frequency of this oscillator varies discontinuously. Therefore, depending on the timing of changing over the frequency dividing ratio, as shown in FIG. 14e, the output pulses of the oscillator 2 skip one to several periods, and the oscillation frequency is low temporarily. If the output voltage Va of the smoothing circuit 13 is at a voltage for obtaining a specified horizontal amplitude at its input frequency as shown in FIG. 14c, the oscillation frequency is lower than the input frequency, and hence the collector pulse voltage generated in the horizontal output transistor 10 is high.
Further, when the power source is cut off at the time of high input frequency, i) the power source may be turned on until the output voltage of the smoothing circuit is not sufficiently lowered, or ii) a high voltage may be applied to the horizontal output transistor when the oscillation frequency elevates at low input frequency.
It is hence necessary to use a horizontal output transistor with the higher maximum collector voltage.
Meanwhile, after the input frequency is changed over, the input frequency is discriminated, and while the oscillation frequency is changed over to be equal to the input frequency, the input frequency and the oscillation frequency of the oscillator are different. Therefore, it may take a certain time until the display is stabilized.
It is an object of the invention to present a deflection apparatus and a control method thereof capable of changing over and controlling in a stable condition, without requiring a high voltage transistor, even if using an oscillator for changing over the oscillation frequency discontinuously depending on the input frequency.
It is another object of the invention to present a deflection apparatus capable of changing over and controlling in a stable condition, even if turning on or off the power source continuously.
It is a further object of the invention to present a deflection apparatus and a control method thereof capable of changing over and controlling smoothly until the input frequency is changed over and the oscillation frequency of the horizontal oscillator is changed over to an optimum frequency.
To achieve the objects, the deflection apparatus of the invention is mainly composed of the following elements.
a) A PWM controlled power supply circuit generates a necessary supply voltage for obtaining a desired horizontal amplitude synchronized with a horizontal sync signal. b) A PWM voltage controller discriminates the frequency of the horizontal sync signal, and controls the output voltage of the PWM controlled power supply circuit. c) An oscillation frequency switching controller discriminates the frequency of the horizontal sync signal, and outputs a frequency changeover voltage to the oscillator so that the oscillation frequency may be optimum for obtaining a specified horizontal amplitude in synchronism with the output voltage of the PWM controlled power supply circuit. d) The oscillator is synchronized with the horizontal sync signal, and outputs a horizontal oscillation pulse at the frequency controlled by the output of the oscillation frequency switching controller. e) The horizontal deflection output circuit is synchronized with the horizontal oscillation pulse, and generates a deflection current for deflecting the electron beam in the horizontal direction.
In this deflection apparatus, for example, when the input frequency is changed from high frequency to low frequency, after detecting the minimum value of the output voltage of the PWM controlled power supply circuit, the oscillation frequency is controlled so as to be an frequency equal to the input frequency. Then the PWM controlled supply voltage is controlled so as to be an optimum voltage for obtaining a specified horizontal amplitude at this input frequency. Accordingly, as the oscillator, by using the oscillator of the type of changing over and controlling the oscillation frequency discontinuously, a high voltage pulse is not generated in the horizontal deflection circuit in the process of changing over the frequency.
Further, assume a case that the oscillation frequency is changed over at a certain changeover point, and then one of the pulses output from the oscillator is missing. Then the horizontal deflection output circuit is driven at a frequency lower than the original frequency. Then the changing over is made at the point where the output voltage of the PWM controlled power supply circuit is at minimum. Therefore high voltage pulse is not generated in the horizontal output circuit even if the oscillation frequency becomes low in the process of changing over the frequency.
Therefore, the horizontal deflection output circuit operates in a stable condition without a high voltage transistor.